This invention relates to a screen printing apparatus and method having a screen and squeegee for forcing a printing material such as ink through the screen to print on a substrate.
The present invention will be described in connection with an illustrated embodiment which is in the form of an oval screen printing machine; but it is not limited to an oval screen printing machine because it is applicable to other forms of screen printing apparatus, such as graphic screen printers, rotary screen printers, bottle printers, etc. A conventional oval screen printing machine typically has a series of pallets connected to a chain for traveling in an endless path through a plurality of printing stations at which are mounted printing heads for printing on the substrates carried by the pallets. Each of the printing heads is lifted at a head stand located outwardly thereof by a lifting cylinder mounted in the head stand. The head stand is electrically connected to a main common controller, such as a programmable logic controller (PLC), which operates the fluid cylinder to raise the print head to allow pallets and the substrates to leave the print head, and to enter the next adjacent print head which is also in the open position. The PLC controller causes the printing heads to lower to their closed position for printing. The opening and closing motions are relatively large movements and are not precisely controlled in their travel speed or easily adjustable as to their length of stroke. The clam shell operation usually allows opening of the head sufficiently to expose the bottom of the printing screen to wipe the same clean, which is a good feature of the clam shell press.
Present oval machines are to a certain extent, expandable from an initial number of printing stations, e.g., from 16 to 20 printing stations, but require expensive and time-consuming operations such as cutting the existing frame and welding on a new frame portion and adding a heavier motor to overcome and move the additional weight of pallets and chains, and friction loads. Typically, the PLC must be reprogrammed; and the entire process involves complex electrical and mechanical operations and connections that are time-consuming and expensive.
During a printing operation, the screen is separated from the substrate by a so-called "peel mechanism" that peels the screen off the substrate to which it is adhered by the printing ink. The peel rate is usually adjustable mechanically in discreet increments often by moving a pin in a lever arrangement to change a mechanical ratio. Such changes in peel rate are done when the printer is stopped and are at a fixed angle or rate once adjusted. The adjustments are relatively large in magnitude. Thus, there is a need for a peel mechanism that is adjustable quickly in small increments without stopping the machine and doing a mechanical adjustment. Further, there is no ability in machines of this kind to do a universal adjustment of the peel rate of several machines simultaneously from a common central controller.
In many conventional screen printers, the length of stroke is controlled by proximity switches that are actuated at limit positions. While the positions of the proximity switches may be mechanically adjusted to change the start and end positions of the stroke, the adjustments are relatively crude. That is, the positions of the limit switches are not very precise, e.g. in 0.001 increments, and are not adjustable to very small displacements of 0.001 inch or the like by a remotely operated controller. In combination with the proximity switches, there are often used shock absorbers, or dashpots, that are used to cushion the stopping of the travel of the squeegee carriage. Proximity switches and shock absorbers tend to have limited life and need to be replaced. The limit switches also preclude multiple print strokes at the same printing station of different stroke lengths. Sometimes, it would be desirable to have different print stroke lengths for a first and second print stroke at the same station. For example, when printing a face on a T-shirt, a light amount of ink may be deposited for printing the face; and a heavy amount of ink may be deposited for the name of the person. It would be desirable to print a light stroke over both the face and name, followed by a short second stroke at the name to deposit more ink over the name, while leaving the face without a second deposit of ink. This is not possible with the mechanical proximity switches and drives currently in use. Further, most controllers are not programmed to provide such a double stroke at the different printing stations.
The amount of off-contact, that is, the spacing of screen from the substrate at the time of printing, is adjustable mechanical by adjusting screws or stops in conventional screen printing presses, such as the above-described oval printing press. Each screen head must be adjusted individually while the head is stationary. Thus, it may be a time-consuming proposition to adjust a single head's off-contact position from a thin substrate such as a T-shirt to a thicker substrate, such as a sweatshirt. It would be preferable that the off-contact distance could be adjusted on the fly and in very small precise increments to either increase or decrease while the machine is operating and done globally, as when switching from T-shirts to sweatshirts.
Another shortcoming of conventional oval machines is the inability to change the print and cure sequences easily because the print heads cannot be easily shifted between stations and without being re-leveled and re-doing their subroutines, of electrically-timed operations with respect to speed, stroke length, peel rate, etc. The shifting of printing heads allows the purchaser of the screen printer to purchase fewer printer heads and the option to later add more printing heads, if desired. The shifting of a head in the common oval printing machine requires a shifting of the head stand and requires a technician to come in and level the print head relative to the platens, thereby defeating a quick, inexpensive change of printing sequence by the shifting of print heads to different printing stations.
A further problem with most current screen printing machines and oval screen printing machines in particular is that each machine is mechanically set up and operated on an ad hoc basis such that it while it may be easy to run the same job later on one screen printer, it is impractical, if not impossible, to run the same job on a second screen printer because the respective screen printers each has been set up on its own ad hoc basis. There is absolute or universal positions or units of operation that allow transmission of the set up variable parameters in absolute values from one screen printer to a second screen printer and achieve the same results. Thus, it is currently difficult to transmit operating machine data from one location to a second remote location to run the same printing job at this second remote location as was run at the first location.
During a set-up operation, it is necessary to accurately register the print screens at a number of print stations. Thus a pallet must be moved from print station to print station to perform such registration. In conventional oval printing machines, a pallet is stepped forward from one print station to the next, with a pause at each station. In a printing machine having a large number of printing stations this results in wasted set-up time when a patent is to be moved more than one print station forward. It would be an advantage if a pallet and a destination print station for that pallet could be identified and the printing machine moved the pallet directly to the destination in the shortest distance, without pausing at intermediate print stations.
Another deficiency in the current oval printing machines and also in many other screen printing machines is the quick change of pallets. In some instances, the changing between overall pallets and standard pallets may take two hours. In many instances, very large and heavy pallets are difficult to secure on their pallet supports and require the use of wrenches for the fasteners used. Thus, there is a need for a faster quick and disconnect of pallets, particularly the larger and heavier pallets.
Additionally, the registration of the substrate for printing is a problem particularly with the larger oval printing machines that may have as many as thirty-six (36) stations. The pallet supports are connected to a chain drive may loosen or become worn with time and allow movement of the pallet supports relative to one another. Because the pallet supports are connected at spaced locations to a chain that goes around a sprocket at each end of the machine, an elastomeric bushing is used at one connection to allow relative movement between the pallet support connections to the chain as one leads the other into and about the curved sprocket path; while the other connection is still traveling along a linear path. The bushing is compressed and then expands in its travel about a curved path. The current conventional machine registers at both the inner and outer edges of the pallet support by discreet registration members that are moved individually by separate actuators into a notch on the respective inner and outer edges, and the pallet support is shifted by compressing the elastomeric bushing. The elastomeric bushing only allows about 1/16" or less shifting of the pallet support, which often, is not a sufficient distance to obtain the registration desired. Thus, there is a need for a registration system that is limited by compressing an elastomeric bushing for the pallet support and is independent of the pallet support unlike the prior art system.